I've compiled all of the data from this thread, along with my own Ultima, Ultima, Shiva, and so far people here have reported:
  • 34 Ultima (25%)
  • 35 Shiva (26%)
  • 40 Lahabrea (30%)
  • 11 Nanamo Ul Namo (8%)
  • 4 Lightning (3%)
  • 5 Firion (4%)
  • 4 Cecil Harvey (3%)
  • 1 Tidus (1%)
(134 total reported)

This seems to imply that you are 10x as likely to get one of the 4*s over one of the heroes, with Nanamo's drop chance somewhere inbetween the heroes and the 4*s. I feel like Tidus's drop rate being so low in this data is just random chance and that it isn't actually lower than the other heroes.